The invention concerns papermachine clothing, and has particular, although not exclusive, reference to clothing for use in the press section of a papermaking machine.
Conventionally, press felts comprise a woven base cloth providing adequate strength in the running direction of the felt and one or more compressible fibre layers applied thereto as, for example, by needling.
Base cloths of the conventional kind are expensive to manufacture and, furthermore, can give rise to the incidence of marking of the paper as it passes through the press section of the papermaking machine due to the presence of knuckles formed at the cross-over points of the warp and weft yarns which form the base cloth.
WO 92/17643 describes a press felt comprising a base cloth made from two superimposed layers of plastics mesh. A batt of fibres is secured to one side of the base cloth by needling. A variation of this felt which has gone into production comprises a base cloth laminate of a plastics mesh with a woven cloth. A batt of staple fibre is needled to the base cloth. Papermachine clothing of this type is prone to delamination particularly when used in press sections of higher-speed papermaking machines where hard press nips are present. This is because the high shear forces resulting from the combination of high-loaded press rolls and the high-speed of the travelling felt tends to cause delamination particularly between the mesh membrane and the woven fabric.
The present invention seeks to provide papermachine clothing, particularly, but not exclusively, for use in the press section of a papermaking machine which is not as susceptible to delamination as these known structures and yet still minimises marking of the paper web supported thereon.
According to the present invention there is provided papermachine clothing comprising a base cloth and a batt fibre layer, said base cloth comprising, at least in part, thermoplastic material in mesh form, wherein said batt fibre layer contains a non woven array of yarns extending in the intended machine direction of the papermachine clothing.
The base cloth preferably comprises at least one layer of woven fabric in combination with at least one layer of thermoplastics material in mesh form The thermoplastics material in mesh form is preferably of the type as described in GB 2202873 and GB 2235705. When a combination of woven and nonwoven mesh material forms the base cloth, the batt and nonwoven array of yarns substantially improves the resistance to delamination between the woven fabric and thermoplastic membrane mesh layer of the base cloth due to the better interlocking of the reinforcing yarns of the batt with the remainder of the fabric. When the base cloth solely comprises mesh membrane material the absence of a woven element to the base cloth eliminates the time consuming complicated and expensive weaving step. Resistance to felt compaction is improved by the use of the mesh membrane and flat parallel nonwoven yarns in the batt layer, the latter presenting a larger contact surface area compared with woven structures when pressure is focussed at the knuckles of the yarn crossover points increasing the likelihood of marking.
It is noted that a non woven array of yarns may also be provided in the batt layer in the cross machine direction in addition to the yams provided in the machine direction. This improves the cross machine direction rigidity of the felt.
The layers of the papermachine clothing are preferably bonded together by needling at least one batt layer of staple fibre onto at least one side of the base cloth.
At least some and preferably all of the reinforcing non woven array of yarns provided in the batt layer are preferably straight and thus non crimped. Prior art non crimped yarn structures in felts have always offered the advantage of providing a greater pressure contact area. Therefore a more uniform press force is exerted on the supported paper web, providing enhanced dewatering with reduced marking tendency. In contrast a woven structure will contain crimped yarns and the pressure is then transmitted to the paper web via the fabric knuckles only. Hence dewatering is more localised, leading to a greater marking tendency. The downside of previous fabrics containing straight yarns is that they lack resilience and the fabrics compact more quickly, resulting in poor dewatering performance. The presence of a nonwoven mesh and the provision of the nonwoven array of yarns in the batt layer on the paper side of the nonwoven mesh together offer vastly improved resistance to compaction and therefore longer working life compared with previous felts containing straight, non crimped yarns.
The reinforcing yarns in the batt preferably comprise a monofilament, multifilament, or twisted cabled yarn (monofilament and/or multifilament component) which may be at least partially encapsulated in and/or may be at least partially impregnated with polymer. Twisted monofilament yarns are preferred. This polymer preferably comprises one or more of the following: polyurethane (e.g. thermoplastic polyurethane), polyester or copolyester, polyamide or copolyamide, silicone or thermoplastic elastomer. The yarns of the non-woven array are preferably coated with an elastomeric polymer, particularly when the optional thermoplastics mesh layer is not present.
The yarns in the batt fibre layer may be prepared by laying elastomer, TPU, coated yams such as monofilaments, multifilaments, cabled yarns or twisted yarns, into profiled grooves in a roll or plate and then apply ultrasonic energy and pressure, separately or simultaneously to the yams to deform the coating material. The cooled yams will preferably have a flat profile, although any shape is possible. This improves the pressure distribution properties of the batt fibre layer into which the yarns are incorporated, improving felt compaction resistance and reducing marking. Noncoated yams may also be deformed by this technique.
It is noted that materials susceptible to heat degradation, eg. nylon or polyolefins, can be used as core yarns. Connecting members may be formed between yarns of an array to form a membrane like mesh, making the non woven array of yams easier to handle when incorporating into the strips of batt fibre material.
The batt preferably comprises one or more nylon fibre materials, optionally with different dtex values. The batt fibres may be predominantly orientated in the machine direction or cross-machine direction, or they may be randomly orientated. The weight of the batt layer(s) is preferably in the range from 300 to 800 g/m2. The dtex range of the batt fibres is ideally in the range from 3 to 70 dtex.
It is noted that the woven layer or layers and the mesh membrane layer or layers may be provided in any arrangement. Typically the layer comprising the batt and non woven array of yarns is secured in place by first needling the batt to the base cloths, typically to a mesh membrane component of the base cloth, and subsequently or simultaneously one or more reinforcing yarns are needled into the batt strip.
The mesh membrane component of the base cloth preferably includes at least one, and ideally a plurality, of load bearing yarns in the machine direction lands thereof. If more than one mesh membrane is provided, ideally, the layers have different mesh sizes. The woven component layer or layers of the base cloth may comprise any weave.